The present invention relates to a camera exposure control device which automatically controls an objective diaphragm aperture in accordance with a preselected shutter speed and the measurement of the scene light having passed through the diaphragm aperture which is being stopped-down. More particularly, it pertains to a camera exposure control device of the above type which is capable of AE-lock photography.
AE-lock photography herein referred to is photography in which exposure is controlled in accordance with stored light measurement effected for a desired portion of a scene to be actually photographed at any desired time prior to, and independently from, an actual camera release operation.
There has been proposed in U.S. Pat. No. 4,103,307 a camera exposure control device of the above type which enables AE-lock photography. Upon selection of a shutter speed priority exposure control mode, the camera exposure control device disclosed in the U.S. patent calculates and stores the step-difference between a fully open diaphragm aperture value and a diaphragm aperture value optimum for the light condition measured for the AE-lock photography. Next, upon actual photographing, the exposure control device measures the brightness of a scene to be actually photographed, through the fully open diaphragm aperture and stores a signal obtained by subtracting the above step-difference from the result of the last mentioned measurement. Subsequentially, the exposure control device measures the brightness of the scene to be actually photographed through the diaphragm which is being stopped-down, and causes diaphragm arresting means to arrest the diaphragm for determination of the diaphragm aperture when, in the course of the stopping-down operation of the diaphragm, the result of this last measurement represents a given relationship with respect to the stored signal obtained by the above subtraction. Thereafter, a shutter is controlled at the preselected shutter speed.
With the exposure control device, the stopping-down operation of the diaphragm is initiated in response to a camera release operation and the determination of the diaphragm aperture is completed for a comparatively short period of time, e.g., 50 m sec.. Hence, the diaphragm blades must be driven at a considerably high speed for the stopping-down operation of the diaphragm. On the other hand, the diaphragm arresting means inevitably has a certain response delay. As a result, at the very moment when the diaphragm arresting means is caused to be actuated, it fails to complete arrestment of the diaphragm, so that the diaphragm is erroneously stopped-down in excess of the optimum diaphragm aperture. It may be possible to preadjust the timing of the actuation of the diaphragm arresting means in anticipation of the amount of the excessive stopping-down of the diaphragm. In fact, however, this would fail to avoid the error in the determination of the diaphragm aperture because the amount of the excessive stopping-down of the diaphragm is affected by a variety of factors and cannot be constant.